Early air-to-surface
radar sets, namely the
ASV Mk. II, had a fairly long minimum detection range. Thus as the aircraft approached the target, it would disappear off the radar at a range that was too great to allow it to be seen by eye at night without some form of illumination. At first aircraft solved this problem by dropping flares to light up the area, but since the flare only lit up the area directly under the aircraft, a string would have to be dropped until the submarine was spotted. Once it was spotted the aircraft would have to circle back to attack, the entire process giving the submarine a fair amount of time to dive out of danger.
Eventually time delayed flares were developed that allowed the attacking plane time to circle. The flare was fired into the air from a buoy previously dropped by the plane. The surfaced submarine could then be seen in silhouette as the plane approached.
Wing Commander Humphrey de Verd Leigh, an RAF personnel officer, came up with his own solution after chatting with returning air crew. This was to mount a searchlight under the aircraft, pointed forward and allowing the submarine to be spotted as soon as it was turned on. He then developed the Leigh light entirely off his own bat, in secret and without official sanction - even the Air Ministry were unaware of its development until shown the completed prototype.
[2] At first it was difficult to fit on aircraft due to its size. Leigh persisted in his efforts to test the idea, and garnered the support of the Commander-in-Chief of Coastal Command, Sir
Frederick Bowhill. In March 1941 a
Vickers Wellington DWI that conveniently already had the necessary generator on board, (it had been used for anti-magnetic mining operations using a large electromagnet) was modified with a retractable "
dustbin" holding the lamp, and proved the concept sound.
At this point the
Air Ministry decided that the idea was worthwhile, but that they should instead use the
Turbinlite, a less effective system which had been originally developed as an aid for nighttime
bomber interception. After trials they too eventually decided to use Leigh's system, but it was not until mid-1942 that aircraft started being modified to carry it.